One patient with congenital adrenal hyperplasia and bilateral testicular tumors was studied. The light microscopy and electronmicroscopy of the tumor were consistent with "Leydig's cell tumor". During control period 24-hour urinary 17-KS, pregnanetriol, and plasma 17-OH-progesterone were elevated. The plasma LH was suppressed in the afternoon samples. The diurnal variation of plasma testosterone was absent. After 48 hours of dexamethasone administration, there was a resumption of periodic pulses of LH (every 90 minutes), and each peak was followed by a peak of plasma testosterne concentration 30 minutes later. These data suggest that adrenal androgens had resulted in suppression of LH during control period. The spermatic vein plasma cortisol was three times peripheral vein level (14.3 mg/dl vs. 5.2) indicating that these testicular tumors secrete cortisol in spite of their morphologic resemblance of Leydig's cells. Dexamethasone administration resulted in reduction in testicular size, while ACTH enlarged the testes, indicating that these tumors had ACTH dependent cells.